While dry conditions amplified this insect in 2002, timely management efforts may help stem the weevil problem in 2003

Talk about salt in the wound: if contending with drought wasn’t enough, a number of sunflower producers this year also battled with sunflower stem weevils.

Referred to also as the spotted stem weevil (Cylindrocopturus adspersus), the insect was most prevalent in the High Plains. A survey of sunflower fields in late September by the National Sunflower Association found weevil larvae in about 70% of fields surveyed in Kansas and Nebraska, and nearly 60% of fields surveyed in South Dakota. Janet Knodel, North Dakota State University extension crop protection specialist, Minot, says there were other hot spots of weevil activity too, including south

central North Dakota.

Damage from sunflower stem weevil larvae that tunnel into stem tissue can result in significant stalk breakage when larval populations are large. There’s also evidence that stem-infesting insects such as stem weevils cause stem wounds that serve as a transmission entrance for disease organisms, further stressing plants. Research indicates that weevil activity may increase sunflower’s susceptibility to phoma black stem, which can cause premature ripening. Stem weevil activity has also been implicated in charcoal stem rot in sunflower in the southern Plains.

The good news for growers is that stem weevils can be controlled without a significant investment.

Stem Weevil Cycles, Scouting

Lodging is a good indicator of stem weevil activity, but bear in mind that plant lodging can also be caused by other factors, such as stalk diameter, stem thickness, sunflower head weight, high wind, and disease. Stalk breakage from weevil activity typically occurs at or slightly above the soil line, in contrast to breakage attributed to disease, which normally occurs farther up on the stalks.

Fields with high stem weevil infestations are a good bet to be hot beds for weevil activity again next year. Weevil larvae overwinter in chambers constructed in the lower stalk or root crown of the sunflower plant. In the spring, larvae pupate in these chambers of infested stalks, from which adults emerge by chewing through the stalk.

Adult sunflower stem weevils emerge from overwintered stalks and root crowns in early to mid-April in the High Plains and mid- to late June in the Northern Plains. Adults are present in the fields in the Northern Plains until late August, with peak densities occurring in mid July. In the High Plains, weevil activity typically subsides by the end of July or early August.

Sampling during the larval stage is difficult, since they develop completely within the sunflower plant. The only method for detecting the presence of larvae is to split the sunflower stem. Adults can also be difficult to scout, due to their small size and “play dead” behavior. They are inactive on the plant or fall to the ground when disturbed and remain motionless. Adults can be found on both upper and underside surfaces of leaves, as well as lower portions of stems, in leaf axils, within dried cotyledons, or in soil cracks at the base of sunflower plants.

Larval population levels of 25 to 30 larvae per stalk (or over 80 per stalk in irrigated sunflower) can result in significant stem damage and lodging - especially under windy conditions, higher plant populations or drought stress. Loss may occur at lower densities when drought conditions cause smaller-diameter stalks.

Since stem weevils appear to prefer earlier planted and more mature plants for egg laying, delayed planting (generally after June 1) may help reduce weevil damage. This tactic should be compared against other ramifications, however; sunflower yield and quality generally is reduced significantly in sunflower crops planted after June 15. Delayed planting isn’t foolproof either. “It didn’t seem to help much this year, since weevils were late in hatching,” says Roger Stockton, Kansas State University extension crops and soils specialist.

Producers should keep in mind as they shop for next year’s seed that some sunflower hybrids are less susceptible to stem breakage than others. Plant population can also influence stalk diameter. Research at North Dakota State University indicated that in sunflower with a weevil population averaging 12 larvae per stalk, lodging was low at both 9,000 and 18,000 plants per acre, while almost 25% of plants lodged when stalk density increased to 36,000 plants per acre.

Treating for Stem Weevils

Planting-time treatments should be considered if a sunflower crop will be planted before June 1, especially if sunflower stem weevil damage was evident in the area during the previous growing season. Carbofuran (Furadan 4F) can be applied at planting, mixed with water or liquid fertilizer, applied directly into the seed furrow.

An in-furrow treatment of Furadan 4F at planting will also help control early-season insect problems, such as wireworms, sunflower beetles and early-season grasshoppers, according to Sam Tutt, northern technical manager for FMC Corp, which makes Furadan 4F. The product is systemic, translocated in the plant through root uptake, with residual effectiveness of about 50 to 55 days, depending on moisture and other factors.

“Hot temperatures will not have much of an impact on Furadan’s residual effectiveness, because it’s in the soil,” says Tutt. “Soil moisture plays a bigger role on residual than ambient temperatures.”

Furadan is offered in returnable 15 gallon “kegs” with an attachable pumping system available from retailers to reduce exposure to the user. Producers can take a big step in maximizing treatment success by following the recommended labeled rate, he says, adding that applying Furadan 4F in-furrow with the seed does not have a negative impact on sunflower seed germination or seedling development.

If early season control measures aren’t taken, producers should scout for weevils—from late June to mid July in the Northern Plains, June into July in the High Plains—and consider a foliar application if necessary.

“For years growers have had success using the scout and treat method, and treating with Furadan. However, under really high numbers and extended emergence like this year, one treatment might not be enough,” says Mike Skolout, FMC representative based in McCook, Neb. “The problem this year was amplified by smaller, weaker plants because of the drought. Hopefully, we won’t have the extreme conditions and such heavy numbers next year.”

Stockton is recommending sunflower producers in the High Plains to include Furadan in their early-season pest control management program for 2003. “For a number of producers, the spotted stem weevil is only half the problem. The soybean stem borer (also called the long-horned beetle, or Dectes stem borer) is also infesting sunflower fields. That insect has multiple generations during the growing season so we might not control all of them, but we’ll control more of them by using a systemic insecticide.”

Foliar insecticides labeled for treatment of stem weevils in sunflower include Asana XL, Warrior, Furadan 4F, Lorsban 4E, Baythroid, Scout X-TRA, and Sevin. With the exception of Sevin, all are restricted use pesticides, and all have a pre-harvest interval ranging from 21 to 60 days.

It's important to begin spraying prior to most of the egg laying, to help ensure control of succeeding larval populations. Banded foliar treatments are effective and economical for row-planted sunflower fields. Economic threshold for treatment is one adult weevil per two plants, or one adult weevil per three plants when stalks are small because of high plant populations or drought stress.

New Stem Weevil Management Bulletin Available

The NDSU Extension Service has published a new bulletin (E-821 revised), “Biology and Integrated Pest Management of Sunflower Stem Weevils in the Great Plains,” by Janet Knodel, North Dakota State University extension crop protection specialist, Minot; and Larry Charlet, entomologist at the USDA-ARS Northern Crop Science Lab in Fargo. The publication can be ordered from the NDSU Extension Service by calling the NDSU Extension Communications Distribution Center, 701-231-7883. The publication can also be found online at www.ext.nodak.edu/extpubs/rowcrops.htm